Current rapid expansion in the use of liquid crystal displays (LCDs), in various areas of information display, is largely due to improvements of display qualities. Contrast, color reproduction, and stable gray scale intensities are important display qualities for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cell, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast, but also causes color or hue shifts with an associated degradation of color reproduction. In addition to black-state light leakage, the narrow viewing angle problem in typical twisted nematic liquid crystal displays is exacerbated by a shift in the brightness-voltage curve as a function of viewing angle because of the optical anisotropy of the liquid crystal material.
Thus, one of the major considerations for evaluating the quality of such displays is the viewing-angle characteristics, which describes a change in contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display devices. One way to improve the viewing-angle characteristics is to insert an optical compensator (also referred to as a compensation film, retardation film, or retarder) situated between the polarizer and liquid crystal cell. An optical compensator can widen the viewing-angle characteristics of liquid crystal displays, and in particular of twisted nematic (TN), super twisted nematic (STN), optically compensated bend (OCB), in plane switching (IPS), or vertically aligned (VA) liquid crystal displays. These various liquid crystal display technologies have been reviewed in U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,410,422 (Bos), and U.S. Pat. No. 4,701,028 (Clerc et al.).
Optical compensators are disclosed in U.S. Pat. No. 5,583,679 (Ito et al.), U.S. Pat. No. 5,853,801 (Suga et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,978,055 (Van De Witte et al.), U.S. Pat. No. 6,160,597 (Schadt et al.), U.S. Patent Application Publication 2002/0041352 A (Kuzuhara et al.), and European Patent Application Publication 1,143,271 A2 (Umeda et al.). A compensation film according to U.S. Pat. No. 5,583,679 (Ito et al.) and U.S. Pat. No. 5,853,801 (Suga et al.), based on discotic liquid crystals that have negative birefringence, is widely used. It offers improved contrast over wider viewing angles, however, it suffers larger color shift for gray level images, compared to a compensator made of liquid crystalline materials with positive birefringence, according to Satoh et al. (“Comparison of Nematic Hybrid and Discotic Hybrid Films as Viewing Angle Compensator for NW-TN-LCDs”, SID 2000 Digest, pp. 347–349, (2000)).
To achieve improved performance in the contrast ratio while limiting color shift, one alternative is to use a pair of crossed liquid crystal polymer films (LCP) on each side of a liquid crystal cell, as discussed by Chen et al. (“Wide Viewing Angle Photoaligned Plastic Films”, SID 99 Digest, pp. 98–101 1999). This paper states that “since the second liquid pre-polymer/LCP retarder film is coated directly on top of the first LCP retarder film, the total thickness of the final wide-view retarder stack is only a few microns thin.” Although this method provides a very compact optical compensator, one of the challenges of this method is to make two LCP layers crossed, particularly in a continuous roll-to-roll manufacturing process.
U.S. Pat. No. 5,853,801 (Suga et al.) teaches a continuous process of preparing an optical compensator by, for example, coating a transparent resin layer on a transparent support, subjecting the layer to a brushing treatment to form an orientation layer, coating a solution of a discotic liquid crystal compound upon the orientation layer, drying the solution of the liquid crystal compound to form a coated layer, and heating the coated layer to form a discotic nematic phase.
In EP 646829 A1, the optical compensator is prepared by a process similar to that described in U.S. Pat. No. 5,853,801 but wherein the steps are performed discontinuously. Thus, the process is not suitable for high volume industrial production.
U.S. Pat. No. 5,995,184 (Chung et al.) discloses a compensation film wherein the film can be removed from a discrete substrate in a discontinuous process that is also unsuitable for high volume industrial production.
U.S. Pat. No. 6,160,597 (Schadt et al.) discloses steps for making an optical compensator using discontinuous or independent steps comprising drying, heating, and cooling treatments for periods as long as one hour or more, and therefore this process is also not suitable for high-volume industrial production.
A problem in the manufacture of optical compensators has been preventing the migration of performance-inhibiting species from the substrate to the optical compensator. Additionally, because of the stringent topographical uniformity requirements associated with optical compensators, the surface of an optical compensator must be free from imperfections typically incurred during manufacture and handling (i.e., scratches, pitting, etc.). Creation of optical compensators on very thin support is also a problem.